Accelerated sintering for a green ceramic sheet

ABSTRACT

A green ceramic sheet material adaptable for accelerated sintering comprising a high alumina ceramic green sheet having an average particle size greater than one micron with a close particle size distribution and being formed into a solid integrated circuit interconnection substrate by heating the ceramic green sheet directly to its sintering temperature, absent the binder burn-off step.

DESCRIPTION OF THE PRIOR ART

In many electronic packaging areas, flexible ceramic green sheetmaterial is employed to form multi-layer ceramic modules. Individualceramic green sheets are metallized, stacked, laminated and fired toform a monolithic ceramic-metal package. This approach provides threedimensional wiring capabilities so as to allow the formation of highdensity electronic packages having excellent performance and reliabilitycharacteristics.

In the prior art, the ceramic green sheet material is formed basicallyby combining a ceramic particulate powder, such as alumina, withsuitable plasticizers, solvents and binders so as to form a slip. Theslip is then cast into thin layers and dried to form the ceramic greensheet material. Greater details of this prior art approach is set forthin the article entitled "A Fabrication Technique for Multi-layer CeramicModules", H. D. Kaiser et al, Solid State Technology, May 1972, pages35-40.

The prior art heating cycle for sintering ceramic green sheet materialis illustrated in FIG. 1. The cycle is constituted by three primaryphases. The organic binder or carbon occupying the interstices is drivenfrom the ceramic green sheet during the binder burn off phase. Next, theceramic particles are sintered at the elevated sintering temperature.Finally, a controlled cool down cycle is employed to prevent thermalshock of the fired solid substrate. In this prior art process, it iscritical that the binder burn-off be accomplished at a very slow andcontrolled rate in order to prevent rupture and explosion due to theoxidation of the binder material and the outward diffusion and releaseof gases. This slow controlled binder burn off is critical in order tomaintain the mechanical and electrical integrity of the fired electronicpackage. The criticality of the binder burn-off step arises by virtue ofthe interstitial make up of the ceramic green sheet material.

In the prior art, the ceramic green sheet material basically comprises ahigh alumina ceramic particulate and glassy frit composition whichdefine a plurality of interstices whose volumes are occupied by theorganic binder of the system. In all prior art methods, the binderburn-off is an extremely critical and important step in the firingprocess. A too rapid heat up causes binder burn-off to occur at a nearlyexplosive rate. Thus, a slow controlled heating phase during the firingcycle is necessary to insure a high quality fired ceramic substratehaving the desired physical and electrical characteristics.

A representative firing cycle for high alumina electricalinterconnection packages as depicted in FIG. 1 illustrates a verygradual binder burn-off phase of approximately 6 hours. During thisportion of the process, the temperature is gradually raised to the finalsintering temperature in the range of 1500° C. over a time period ofapproximately 6 hours. The alumina particles are then maintained at thiselevated temperature for a period of approximately 3 to 7 hours in orderto sinter the alumina particles. Thereafter, a slow controlled cooledslow down step is necessary in order to prevent thermal shock.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a flexibleceramic dried green sheet material which can be sintered without acritical binder burn-off step to produce high alumina electricalinterconnection substrates.

In accordance with the aforementioned object, the present inventionprovides a flexible ceramic dried green sheet dispersion comprising highalumina particles having an average particle size greater than onemicron with attendant close particle size distribution which can besintered by heating it directly to its sintering temperature without acontrolled slow binder burn-off step.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a curve illustrating a prior art firing cycle comprising acritical binder burn-off heating cycle, a sintering cycle, and acontrolled cool down cycle.

FIG. 2 is a process block diagram illustrating the basic steps forforming the flexible ceramic green sheet material of the presentinvention.

FIG. 3 depicts a firing cycle for the present invention and illustratesthat the binder burn out step is substantially eliminated.

FIGS. 4, 5 and 6, 7 are greatly exploded plan and cross-sectional viewsof a ceramic green sheet material schematically illustrating thecomposition make up before and after firing, respectively.

FIG. 8 is a curve illustrating the particle size distribution necessaryfor forming the ceramic green sheet of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Now referring to FIG. 2, it illustrates the general process steps forforming the ceramic green sheet material of the present invention.Initially, the particle sizes of a high alumina powder and the glassyfrit are adjusted at 10 in a raw milling step by conventional means suchas ball or vibrator milling. These constituents are then dried at step12. Specific details of the raw material milling step as it pertains toparticle size distribution are discussed in greater detail below, andonly the general steps necessary to form the green sheet material areillustrated in FIG. 2. A suitable organic binder is formed at step 14.In actual practice, the binder also comprises an organic material, suchas, polyvinyl butyral, a plasticizer such as dioctyl pthalate, and anorganic volatile solvent, such as toluene-alcohol and cyclohexanone, asis well known in the art.

High alumina, Al₂ O₃, ceramic powders are commercially available andessentially comprise 99.6% alumina Al₂ O₃ and 0.04% NA₂ O, by way ofexample. One suitable glassy frit composition for the preferredembodiment substantially consists of:

*Ceramitalc: 23.72 wt %

**Kaolin-Ajax P: 62.77 wt %

***Cab-O-Sil: 2.66 wt %

CaCo₃ : 10.83 wt %

The slip material is formed at 16 and then cast into thin sheets. Nextit is dried for forming the finished ceramic green sheet illustrated atstep 18. The dried ceramic green tape is punched at 20 to form thedesired interconnection via hole patterns when the ceramic green tape isused to form a multi-layer ceramic interconnection package. Any desiredmetallized electrical interconnection pattern (single or multi-layerapplications) is deposited on the ceramic green sheet layer, as forexample by a screening at 22. A plurality of punched and screenedsheets, one of which is structurally illustrated at 24, are thenlaminated and fired at steps 26 and 28, respectively, in order to form ametallized multi-layer ceramic interconnection electrical package.

Now referring to FIGS. 3 through 8 for a more detailed description ofthe raw material milling step and the firing step generally described inthe schematic process steps of FIG. 2 at 10 and 28, respectively. Duringraw material milling, it is necessary to form predetermined preciselysized alumina particles in order to allow subsequent firing directly toa sintering temperature without the controlled binder burn-off step. Thesolid curve 35 in FIG. 8 illustrates the necessary and critical particlesize distribution. The curve between points 36 and 38 illustrate thatapproximately 95% of the particle sizes used to form the ceramic greensheet of the present material is between 1 and 10 microns. Further, ithas been found that it is necessary that the average particle size ofthe high alumina starting material must be greater than 1 micron, andthis limitation is represented by the mean point 40 on the soliddistribution curve. From the solid distribution curve 35, it is apparentthat 95% of the particle sizes fall within 1 or 2 microns of the averageor mean value represented by point 40. Dotted distribution curve 41 and42 illustrate particle size distributions on both ends of the spectrumwhich in actual practice are unsuitable for applicants' invention. Thefrit is milled with the alumina and because it is softer than thealumina, the milling process reduces the frit particle size distributionto essentially that of the alumina.

As seen in FIGS. 4 and 5, the green sheet material 43 is constituted bya plurality of alumina particles 44 and glassy frit particles 46 in anumerical weight ratio of approximately 10:1, and of comparable sizesince the frit and alumina are milled together. In combination theycreate a plurality of interstices whose volumes are defined primarily bythe average particle size distribution of the alumina particles 44 andthe glassy frit. Prior to firing, the plurality of interstices, showngenerally at 48, are occupied by the organic binder materialschematically shown at 50. By carefully controlling the particle sizedistribution, substantially represented by curve 35, the plurality ofinterstices 50 are of sufficient volume so as to enable rapid andaccelerated binder removal as the green ceramic sheet is raised directlyto its sintering temperature. A particle size distribution asillustrated by the dotted curve 42 of FIG. 8 results in minute particlesof the alumina itself occupying the interstices formed by the largerparticles. This condition causes the ceramic green sheet material toerupt or explode if raised directly to its sintering temperature. On theother hand, an excessively large particle size, as represented by thedistribution curve 41, is undesirable and necessitates an extremely highsintering temperature substantially above the 1500° range to thatassociated with the present day state of the art high aluminasubstrates. Moreover, the extremely high sintering temperatures aredetrimental to most metallurgical systems used in the formation of highalumina electrical interconnection substrates.

One representative firing cycle curve 51 for ceramic green sheetsfabricated according to the present invention, with the binder burn outstep substantially eliminated, is illustrated in FIG. 3. The temperatureinitially is raised from approximately 20° C. to 1500° C. inapproximately 30 minutes. The ceramic green sheet is then held at itssintering temperature for approximately 3 hours, and thereafter cooledto ambient conditions in approximately 1/2 hour. Significantly, ceramicgreen sheet material formed in accordance with the present inventionpreferred embodiment is readily and successfully fireable to itsassociated sintering temperature at initial rates of 50° C./minute.However, ceramic green sheets also have been successfully fired underinitial heating cycles in the range of 400° C. per minute.

After firing, the ceramic green sheet material is sintered to produce asolid substrate as schematically illustrated in FIGS. 6 and 7. The firedsubstrate now comprises a plurality of sintered ceramic particles 60 anda homogeneous glassy frit material 62 occupying the interstices of thestructure. The organic binder previously contained in the ceramic greensheet material, illustrated in FIGS. 4 and 5, is rapidly oxidized duringthe initial heating cycle when the ceramic green sheet material is beingraised directly to its sintering temperature.

The fired alumina particles illustrated in FIGS. 6 and 7 are shown asbeing of slightly irregular shape compared to that illustrated in FIGS.4 and 5, as a slight chemical reaction occurs between the glassy fritand the alumina particles during the sintering portion of the heatingcycle, approximately 1500° for the specific compositions of materialsillustrated in the preferred embodiment of the present invention.

Although the FIG. 3 embodiment illustrates an initial heating rate of50° C. per minute as constituting a preferred rate, it is to beunderstood that success in firing ceramic green sheet materials has beenachieved at initial rates of 400° C./minute up to a temperature of 1400°C. in dry and wet forming gas having a 25° C. dew point, without anydelamination or cracking of parts.

Other ceramic green sheets, comprising a high alumina ceramicparticulates and a glassy frit substantially as previously described inconnection with the preferred embodiment of the present invention, havebeen successfully fired in a 25° C. dew point forming gas. The ceramicgreen sheet material is subjected to an initial heating rate of 50°C./minute going from 20° C. to 1530° C. in 30 minutes and then it isheld at its sintering temperature of 1530° C. for approximately 1 hour.Thereafter, the structure is cooled from 1530° C. to 20° C. in 30minutes. Successful results have been achieved with parts ranging fromdimensions of 0.67 inches by 0.67 inches by 0.084 inches to 1.008 inchesby 1.008 inches by 0.865 inches. For these particular samples, theresulting fired ceramic solid body possessed fired densities in thegeneral range of 3.75 grams per cubic centimeter and shrinkage rates ofany where between 13.2% to 15.5%. These dimensions are onlyillustrative, and the invention is equally applicable to larger parts aslong as thermal shock is avoided.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A solvent-free flexible ceramic dried green sheetdispersion comprising:a. a plurality of ceramic particles having anaverage particle size represented by point 40 on the graph of FIG. 8with 95% of said particles falling within 2 microns of said point, b. incombination with a glassy frit material, and c. a pyrolyticallydecomposable polymeric organic binder dispersed with said plurality ofceramic particles and said glassy frit material and occupying theinterstices therebetween.
 2. A flexible ceramic dried green sheetdispersion as in claim 1 wherein:a. said ceramic particles comprisessubstantially pure alumina, Al₂ O₃.
 3. A flexible ceramic dried greensheet dispersion as in claim 1 wherein said frit has a particle sizedistribution essentially that of said ceramic particles.
 4. A flexibleceramic dried green sheet dispersion as in claim 2 wherein said frit hasa particle size distribution essentially that of said ceramic particles.